1. Field of the Invention
The present invention relates to an optical scanning apparatus mounted on an image forming apparatus such as a copier, printer, and facsimile machine for exposure-scanning surfaces of image bearing members with optical beams (e.g., laser beams).
2. Description of the Related Art
Optical scanning apparatuses used in copiers, printers, and the like generally expose a surface of an image bearing member typified by a photosensitive drum, i.e., a scanned surface, while scanning it, and thereby form a predetermined electrostatic image on the surface of the photosensitive drum. In optical scanning apparatus, an optical beam, such as a laser beam, emitted from a light source is deflected in a main scanning direction by a light deflector and directed at the scanned surface by an optical system.
Recently, image forming apparatus capable of forming color images have spread widely. They include so-called tandem image forming apparatus which are equipped with photosensitive drums for multiple colors. The tandem image forming apparatus generally have four photosensitive drums corresponding to four colors of yellow, magenta, cyan, and black lined up in order along a sheet transport direction or along a travel (rotation) direction of intermediate transfer bodies. On the tandem image forming apparatus, laser beams emitted from an optical scanning apparatus form electrostatic images corresponding to the four colors on respective photosensitive drums, the electrostatic images are developed to toner images of the respective colors, and the toner images are superimposedly transferred to a transfer sheet to form a color image.
An example of an optical scanning apparatus mounted on such a tandem image forming apparatus is found in Japanese Patent Laid-Open No. 2006-91346 (D1). The optical scanning apparatus (optical beam scanning apparatus) described in Document D1 has light sources corresponding to four colors of yellow, magenta, cyan, and black. The laser beams emitted from the light sources enter a polygon mirror, which is a light deflector, at different angles from each other. The polygon mirror reflects the laser beams off its reflecting surfaces and thereby deflects them in the main scanning direction. The deflected laser beams are further deflected parallel to the axis direction of the photosensitive drums at uniform velocity by an fθ lens and directed at the surfaces of the photosensitive drums via reflecting mirrors to form an image.
The optical scanning apparatus (optical beam scanning apparatus) mounted on a tandem image forming apparatus such as the one disclosed in Document D1 is equipped with an optical sensor to receive the laser beams reflected by the polygon mirror excluding that part of the optical beams which falls in an effective exposure area of the scanned surface and is used to control scan timing for the laser beams corresponding to the four colors. According to D1, to enable accurate detection, the laser beam with the smallest scanning curvature (curvilinear distortion of scan lines) is used out of the laser beams passing through the fθ lens.
However, depending on the optical scanning apparatus, the laser beams reflected by the polygon mirror are sometimes configured to enter the optical sensor directly without passing through the fθ lens, by force of the structure of an inner optical path. In that case, it may not be possible to avoid adverse effects on scanning which are caused by incident angles of laser beams on the polygon mirror, face tangle error of the polygon mirror, and the like and which should essentially be corrected by the fθ lens. Also, when correcting scanning curvature using two or more fθ lenses for convenience of placement of the optical system, such a level of laser beams that cannot be corrected sufficiently may enter the optical sensor. In these cases, the laser beams enter the optical sensor, maintaining scanning curvature, and thus it is highly likely that a deviation will occur in the scan timing for the laser beams corresponding to multiple colors. This can cause out-of-color registration of a color image, resulting in degradation of image quality.